1. Field of the Invention
The invention relates generally to control systems for motor operated apparatus, and is concerned more particularly with a control system for efficiently rotating the anode of an x-ray tube at a speed commensurate with selected operating parameters of the tube.
2. Discussion of the Prior Art
A rotating anode x-ray tube may include a tubular envelope wherein an anode target disc is disposed transversely for rotation about its axial centerline. The target disc generally has a frusto-conical surface disposed adjacent an electron emitting cathode, which may be of the filamentary type, for example. The sloped outer peripheral portion of the frusto-conical anode surface constitutes a focal track having aligned with the cathode a focal spot area of constantly changing focal track material when the anode disc is rotating. The focal track includes material, such as tungsten, for example, suitable for readily emitting x-rays from the focal spot area in response to impinging electrons beamed from the cathode. The useful x-rays emitted from the sloped focal spot area pass in a beam through a radially aligned window in the tube envelope.
The anode target disc generally is rotated by a rotor of an AC induction motor axially disposed in an end portion of the envelope, which is encircled by an external stator of the motor. The x-ray tube and encircling stator usually are insulatingly supported in an x-ray shielded housing, which may be filled with a transparent dielectric coolant, such as oil, for example. The housing generally has an x-ray transparent port aligned with the window in the tube envelope to permit egress of the x-ray beam from the housing. Also, the housing usually is provided with a spaced pair of conventional horn-type connectors, whereby external sources of electrical energy connected through suitable control units are, in turn, connected to appropriate electrodes of the x-ray tube.
In operation, the cathode initially is heated electrically to a standby temperature, which generally is considerably less than the desired operating temperature of the cathode. Just prior to taking an x-ray exposure, the cathode is heated to an incandescent temperature sufficient to supply the desired electron current during a selected exposure interval. Simultaneously, the AC induction motor is electrically "boosted", or accelerated, to a speed sufficient to protect the material of the anode target disc from overheating during the exposure interval. Generally, a predetermined fixed time interval is allowed for the cathode to heat to the desired temperature and for the anode disc to accelerate to the required speed. When the fixed time interval has elapsed, a high voltage is applied between the cathode and the anode of the x-ray tube to electrostatically beam electrons emitted from the cathode onto the focal spot area of the anode with sufficient energy to generate x-rays, which radiate from the focal spot area in all directions. The useful portion of the x-rays pass in a beam through the x-ray transparent window in the tube envelope and egress from the aligned port in the housing.
However, only about one percent of the electron energy impinging on the focal spot area of the anode disc is converted into x-ray energy. The remaining ninety-nine percent of the electron energy is converted into heat which must be dissipated by the anode target disc. Consequently, the anode target is rotated at a speed sufficient to ensure that successive discrete areas of the focal track pass rapidly through the electron beam in order to prevent overheating which causes vaporization or cracking of the focal track material. However, the anode target speed should allow sufficient time for the successive discrete areas of the focal track to dissipate heat before being rotated through the electron beam again. Thus, the anode target should be rotated within an optimum speed range dependent on the operating parameters of the tube.
Generally, the fixed time delay commonly used to ensure the anode disc has attained the required speed has proved unsatisfactory because it does not take into account variations in the structure of the tube. Also, other prior art means for monitoring anode disc speed, such as vibration and acoustical sensing devices, for examples, may provide inaccurate measurements due to the effects of extranneous signals unrelated to the anode speed. Furthermore, prior art means for monitoring anode disc speed generally are used to measure restricted speed limits, such as 3000 RPM for fluoroscopy and 9000 RPM for radiography, for examples. Thus, these prior art measuring systems do not provide means for determining that the anode rotational speed is suitable for the operating power parameters selected for a particular x-ray exposure.
Therefore, it is necessary and desirable to provide a rotating anode tube with a control system having means for ensuring that the anode disc is rotating at a speed commensurate with selected operating parameters, and in a manner independent of ambient energy signals.